The process of photographic product making is a complex procedure about which a lot has been disclosed and patented. In general terms the process to manufacture a photographic product like a photographic paper or triacetate cellulose film consists of coating several layers on top of either a laminated paper or a transparent polymer support. These layers are known as emulsion layers which can contain the radiation sensitive silver halide crystals as the most essential component or intermediate layers without these photosensitive components. The subject invention is directed at improving the photosensitive layer as such and improving the production process of photographic layers.
There are several stages at which gelatin is used in the process of film making. The function of the gelatin is different in each stage and thus the required characteristics for each stage are different and it is to be expected that collagen like substances can be specifically tailored to suit each particular application.
A lot of attention has been focussed on the process of making silver halide emulsions for photographic applications. A lot of attention has been paid to the role of grain morphology of silver halide crystals and aspects that influence the AgX nucleation process and the subsequent ripening process. The most essential component in an emulsion layer of a photographic product consists of radiation sensitive silver bromide, silver chloride or silver bromochloride microcrystals optionally containing iodide which are commonly referred to as silver halide grains. A peptizer is introduced during the precipitation of the grains to avoid uncontrolled coalescence which coalescence will otherwise exhibit a number of disadvantages i.a. limit the formation of thin intermediate and high aspect ratio tabular grain emulsions which in turn is disadvantageous in photography. Gelatin in numerous forms has been used in photographic manufacturing processes as peptizer. It is well known that the tabular grains with high aspect ratio have several photographic advantages like increased sharpness, improved speed granularity relationships, increased blue and minus blue speed separation, more rapid developability and higher silver covering power (Research Disclosure Vol. 225 January 1983, Item 22534; EP-A-0.610.796). It has also been desired to produce tabular grains not only with high aspect ratio but also with a narrow grain size distribution otherwise expressed as a desire for mono or homodispersity.
To date the gelatin used in commercial processes has been derived from animal sources in general simply by derival from animal bone and hide. The disadvantages of this material are the presence of impurities and the fact that the nature of the composition is unclearly defined and thus not reproducible. It is unclear what components are present and in which amounts. In addition it is also unclear which components actually are required for optimal activity. The reproducibility of the photographic manufacturing process is questionable due to the lack of consistency of the gelatin composition used at various stages of the photographic manufacturing process.
The disadvantages of gelatin in photographic applications have been addressed in detail over the years and have been the subject of various patent applications. Most of these documents have been directed at addressing processes of developing modified gelatins after the derival thereof from the animal source to introduce particular improvements in characteristics of the modified gelatins. In 1984 U.S. Pat. No. 4,439,520 describes a desirability for more than 50% of the crystals to have aspect ratios higher than 8 as this would increase blue speed. In 1987 U.S. Pat. No. 4,713,320 mentions using gelatin with a methionine content below 30 micromoles per gram, preferably less than 5 micromoles per gram to arrive at thin trapezoidal, hexagonal and triangular tabular grains. A normal bone derived gelatin was used which had been oxidized in order to achieve a level of methionine below 30 micromoles per gram of gelatin. The lower methionine content is also described in U.S. Pat. No. 4,914,014 in 1990 as offering a wider range of pBr during precipitation conditions. Numerous publications cover processes for reducing the methionine content of gelatin. EP-558.410 published in 1993 describes oxidizing reagent reaction of alkali hypochlorite or H.sub.2 O.sub.2 as do articles in J.Photo.Sci 40 230--230 (Nippi), J.Photo.Sci 37, 14-18 (AGFA) of 1992 and J.Imag.Sci 33,1 3-17 of 1989. Even as early as 1959 oxidation was suggested as a manner to remove impurities.
There has also been a lot of research carried out on collagen and collagen like proteins per se using recombinant nucleic acid technology. The use of recombinant DNA technology in combination with collagen and the application thereof in photographic application has however been remarkably absent. Most of the documents published in the area of recombinant collagen have been directed at diagnostic applications using PCR technology on genomic nucleic acid not even requiring expression of the collagen encoding sequence. The mere presence of the sequence in the genome suffices for diagnosis in these instances. Any such documents actually mentioning expression of collagen encoding sequences certainly have not required a high degree of expression. Alternatively where expression is mentioned merely portions of the encoding sequence are expressed rather than the complete sequence. Often these partial sequences are used for eliciting antibodies for which amounts of proteinaceous material required are minimal. In addition once the antibodies are obtained the sequences are not required further for pharmaceutical application. Therefore in these instances low expression is not a relevant issue. Also the expression of small portions of the encoding sequence can be expected to eliminate expression problems which are attributed to the high degree of repetitivity of the encoding sequence.
Synthetic nucleic acid has been designed in an effort to overcome expression problems associated with longer repetitive sequences and also in an effort to design new types of protein i.e. synthetic protein. Such synthetic polypeptides are however extremely expensive to produce. It is thus not feasible to apply such in applications requiring large scale production such as required in the field of photographic film production.
Most applications of the prior art thus either do not require the high degree of expression required for production on an industrial scale or do not in fact provide the desired result. The documents discussing these types of application have consequently either not addressed or have not solved the problem of obtaining high expression of native collagen sequences or sequences of corresponding length and structure.
In general where the prior art has suggested expressing native sequences or parts thereof general terms have been used and referrals to handbooks for general transformation protocols have been given without further detail. Any examples provided have used E. coli or S. cerevisiae as producing organism and degree of expression has been of minor importance or has not been focussed on.
Where expression problems of collagen like protein have been addressed this has occurred by using either modified E. coli or higher animal cells and insect cells. The latter are also modified for post translational processing. The application of the latter type of cells is however prohibitive for large scale production due to the high costs of the cells, the media and the isolation of product. The disadvantage of E. coli is that it cannot secrete the desired product. In addition the repetitivity of a nucleic acid sequence to be expressed will provide instability of the transformed bacterium and thus result in low expression levels for any collagen like encoding sequence. It is thus not feasible to apply such production (micro)organisms in applications requiring large scale production as required in the field of photographic film production.
A lot of research effort has been directed at achieving the post transcription modification required to arrive at fibrillar or triple helix collagen which is the state of collagen as present in animal sources i.e. the state of collagen currently applied in industrial scale photographic applications. It has been generally accepted that host cells comprising post translational processing apparatus as such themselves or through addition of encoding sequences for post translational processing enzymes should be used when expressing collagen like material in order to arrive at collagen with a triple helix and more particularly to arrive at fibrillar collagen. It is commonly accepted that this form of collagen is the useful form for application.
Where the prior art has actually tentatively actually suggested recombinant collagen could be used in photographic applications the relevance of the particular form of collagen material has usually not been addressed vis a vis the requirements specific for such application. Some patent applications have mentioned in passing the use of recombinant collagen for photographic film, some even specifically mention photographic application. The teachings of such documents are clearly however directed at other issues and are not directed specifically at photographic applications and the special requirements thereof. Closer analysis actually shows that for various reasons none of the examples provided in such applications are in fact even suited for application in photographic films. The patent applications are considered non enabling and merely speculative of nature when it comes to applying recombinant collagen in photographic manufacturing processes. Examples of such documents are now provided.
WO90/05177 describes the production of novel polymers comprising small repeating sequences, specifically repeating groups such as silk are disclosed. Collagen is suggested as one of the structures capable of providing a repeating unit. It is stated "The properties of CLP were designed so they would undergo thermoreversible gelation at high temperatures as well as being non immunogenic. The high stability of the helices should create high tensile strength in fibres or membranes formulated from CLP. "These chain properties should allow the creation of hydrogel colloids in aqueous solutions which when applied to hard substrates should act as soft coatings." A suggestion is then given of a soft coating material with a ligand for a cellular receptor. The sequence (SEQ ID NO: 1) GLPGPKGDRGDAGPKGADGSP was to be added to the CLP monomer and an example of a construct to be expressed from E. coli is provided. With regard to this composition it is disclosed. "The subject compositions may find use as wound dressings, allowing for neovascularisation, eye applications, matrices for artificial organs and the like." The combination of CLP with other repetitive functional units thereby combining functions is also suggested. However no examples are provided of sequences used.
The only examples provided show a recombinantly produced synthetic CLP polypeptide (SEQ ID NO: 2) [[GAP(GPP)3]2[GPVGSP]n with N-terminal and C terminal spacers. The spacers are 33 amino acids and 25 amino acids in length. Thus the repetitive GPP portion of the polymer which is 24 amino acids in length is separated by 33+25+6 amino acids. In this manner E coli apparently managed to express a CLP protein of 760 amino acids, i.e. MW 63.800. The cell binding CLP had the same basic structure but the hexamer was replaced by the cell binding sequence given above resulting in an amino acid length of 814 amino acids and a MW of 70.560. The repetitive GXY motif that is expressed is short and is separated by long none repetitive sequences. The spacer DNA encodes 2 cysteine residues and also 3 methionine residues.
The cited document states in the introductory part concerning collagen "Chemically hydrolysed natural collagen can be denatured and renatured by heating and cooling to produce gelatin which is used in photographic and medical applications among other applications. The chain property of collagen responsible for this phenomenon is its ability to spontaneously form interchain aggregates having a conformation designated as a triple helix." It is thus particularly remarked in this prior art document that helical structure was required. The subsequent text is actually silent on any photographic applications and is clearly directed at completely other matters. The subsequent text is also silent on actual degree of expression obtained by E. coli. The repetitive structure is present to such a low degree it is unlikely to retain sufficient collagen like activity to be useful in photographic application. In addition the presence of cysteine and methionine residues at the levels provided herein in the expression product in fact render such inappropriate for use in AgX emulsions for photographic applications. Furthermore it is unclear whether the use of the less repetitive sequences as described here actually provided any improved level of expression in E. coli. Thus a person skilled in the art of photographic applications would be dissuaded from applying the teaching of this document in photographic applications. Firstly because it is unclear whether industrial scale production would be feasible. Considering instability of repetitive sequences this is unlikely. Secondly it is unlikely due to the undesired presence of cysteine and methionine in AgX emulsions for photographic applications. Thirdly this is unlikely due to the absence of helical structure of the expression product. The impact thereof is totally unpredictable vis a vis stability of expression product and vis avis applicability in photography considering the major structural difference of current gelatins.
The same inventors as the preceding cited patent application disclose in WO93/10154 high molecular weight collagen like protein polymers having repetitive triads with reduced proline content. They are stated as being capable of production in unicellular microorganisms at high molecular weights and at high efficiency. They indicate. "The uniqueness of collagen repetitive tripeptide is a challenge for recombinant technology in light of the high repetitiveness of the sequence and the frequent utility of the amino acids glycine and proline in the composition. Genes encoding proteins with high levels of glycine and proline are by necessity composed of high levels of the nuclcotides guanidine and cytidine in self complementary sequences. Thus as one synthesizes the gene there is substantial opportunity for strands to loop out, single stranded DNA to be excised, recombination events to occur which can result in loss of segments of the gene and inefficient transcription and/or translation. Thus there is substantial interest in developing techniques and compositions which provide the advantageous properties of collagen while at the same time allowing for stable expression of high molecular weight collagen like proteins." In addition it is stated. "The polymers will further be characterised in, being like collagen, providing helices, capable of denaturation and renaturation, forming gels etc." A molecular weight between 30-150 kD is suggested and at least 45 number % of the amino acids between the glycines are proline, at least 80 weight % of triads have glycine as first amino acid, at least 40% by number of the triads comprise at least one proline. The example shows use of 3 types of repetitive GXO encoding sequences and N terminal and C terminal spacer sequences. The same spacer sequences as in the previous patent application were used. The structure of the repetitive sequences was (SEQ ID NO: 3) [[GAHGPAGPK]2(GAPGPAGPP)24(GAHGPAGPK)2]2=[[C]2[A]24[C]2]2. The length of the polypeptide produced was 561 amino acids with a MW of 46.409 Dalton. In another example the repetitive sequence was (SEQ ID NO: 4) [[GAHGPAGPK]2(GAPGPAGPP)12(GAHGPAGPK)2]5=[[C]2[A]12[C]2]5. The length of the polypeptide produced was 777 amino acids with a MW of 64.094 Dalton, with an observed protein band at 100 kD. In the third example the structure was (SEQ ID NO: 5) [[GAHGPAGPK]2(GAPGPAGPPGSRDPGPP)12(GAHGPAGPK)2]4=[[C]2[AB]12[C]2]4. The example had 1065 amino acids and MW 91,966 with a protein band visual at 135 kD. Apparently smaller versions were also produced with protein band weights of 28 kD, 64 and 98 kD. With regard to expression the only details provided are that detection by western blot with antisera was carried out and that the expression of the full length polymer decreased with gene size, whilst the synthesis of full length mRNA was at equivalent levels. Another group of polymers with two other different repetitive units were produced [[C]2[DB]12[C]2]4, (SEQ ID NO: 6) [[C]2[DB]6[C]2]4 (SEQ ID NO: 7) and [[C]2[D]24[C]2]4, (SEQ ID NO: 8) wherein B and C are as above and D=GAQGPAGPG (SEQ ID NO: 9). Respectively these 3 proteins illustrated had 1065 amino acids and MW 91,533 D, 633 amino acids and MW 55,228 D, 1065 amino acids and MW 85,386 D with a protein band visual at 140 kD. Of the examples the only information concerning characteristics of the product are provided for number 6. This product is extremely soluble in water. At room temperature or above solutions thereof of more than 8% in water are viscous but they are fluid and form to a solid gel upon chilling to 0.degree. C. Upon heating above 28.degree. C. the gel forms a thick solution. A thermoreversible transition between liquid and gel is thus illustrated. The final example concerned a structure (GAPSQGAPGLQ)68 (SEQ ID NO: 10) also with the same spacers and 1077 amino acids and a MW 91.266 D. With regard to application of such polypeptide nothing more is stated than in the previously cited application of these inventors. Apparently by varying the block copolymer structure of the repetitive GXO motif it has become possible for expression of longer repetitive sequences to occur. How efficient such protein is expressed is however not clear. Yet again the expression problem due to repetitivity is not illustrated as being solved. It is questionable industrial scale expression could be achieved. No teaching specifically concerning photographic applications is provided. In particular all examples use spacer with cysteine and methionine which is undesirable in photographic applications. Thus a person skilled in the art of photographic applications would be dissuaded from applying the teaching of this document in photographic applications.
French patent 2685347 discusses the desirability of producing recombinant material similar in properties to gelatin. The advantage would be a more homogenous product solving reproducibility problems and the chance to modify chemical functions thereof. The idea is to produce oligopeptides as gelatin substitutes, The microorganism selected was E. coli and it is stated the absence of post translation modifications such as glycolisation common to coli is no problem. Other hosts are said to be possible. No examples are however given of such possible hosts. The nucleic acid sequence to be applied must comprise a gelatin peptide encoding sequence for (Gly-X-Y)n linked to Met-Cys-His-His-His-Leu-Met (SEQ ID NO: 11) codons in order for selection to occur. The sequence given by way of example encodes (SEQ ID NO: 12) Gly-Pro-Ala-Gly-Glu-Arg-Gly-Pro-Lys-Gly-Trp-Met. In a later thesis by the inventor it became apparent that the degree of expression was in fact found to be inadequate for any kind of industrial application. In addition the retrieval process of the produced amino acid was complicated. E coli was the host cell described and obviously presented as a matter of fact the disadvantages already presented above for a person skilled in the art interested in industrial scale production i.e. lack of secretion, instability of repetitive sequences and thus a low degree of expression. Finally nothing was actually illustrated concerning application of the suggested collagen in a photographic film in this prior art document. Thus a person skilled in the art of photographic applications would be dissuaded from applying the teaching of this document in photographic applications due to unpredictable outcome of such structurally different material.
Finally a U.S. Pat. No. 5,580,712 isssued to Eastman-Kodak in 1996 concerned with specificially modified collagen like polypeptides and application thereof for photographic purposes describes that collagen like peptizers with silver binding strengths below 50 mV can lead to a high degree of thin tabular grain. The document illustrates this for a number of synthetically produced polypeptides with a length of 25 amino acids. The document also mentions one polypeptide with a collagen like structure was produced using recombinant technology. The recombinant polypeptide is a synthetic polypeptide of block copolymer structure consisting of merely 4 different amino acids. No actual expression details are provided for this recombinant polypeptide, a reference is merely made to standard molecular biology production protocols and the use of Saccharomyces cerevisiae as expression host. The molecular weight is approximately 26 kDa. It thus is questionable whether a molecular biologist familiar with the expression problems provided in detail in other documents of the same date and later would seriously contemplate such production. In addition no details concerning binding strength of the recombinantly produced product are provided thus it is also questionable whether a person skilled in the art of photographic applications would seriously consider use of this product in a silver halide emulsion for photographic product or would seriously expect it to exhibit the characteristics of the short synthetic polypeptides disclosed. The document also suggests that the specific polypeptides disclosed comprising histidine and methionine at specific points namely at Xaa of the following formula will exhibit high binding strengths and will exhibit non tabular grain formation. The formula of the compound is (SEQ ID NO: 13) Gly Pro Xaal Gly Leu Xaa2 Gly Pro Arg Gly Pro Pro Gly Ala Ser Gly Ala Pro Gly Phe Gln Gly. Analysis of the Table providing details of the compounds researched by Eastman-Kodak reveals that all compounds with high binding strengths had at least one reducing amino acid (=histidine or methionine) per 25 amino acids thus resulting in contents higher than 400 micromoles of methionine per gram of polypeptide. Such compounds will not be useful in nucleation/growth AgX-emulsion processes for photographic applications. The synthetic compounds illustrated as having low binding strength and favouring tabular grain formation did not comprise any reducing amino acids Met or His. A number of other US patents are issued to Eastman-Kodak on related subject matter. These patents (U.S. Pat. No. 5,580,712 and U.S. Pat. No. 5,670,616) revealed other synthetic fragments purported to be useful for tabular grain formation but the same single recombinant product example is described so these patents offer nothing new vis a vis recombinant collagen expression and application of recombinant collagen in photographic applications with AgE-emulsions. It is also derivable from these descriptions of Eastman-Kodak that expression of the specific sequence shown which occurred in the host S. cerevisiae was in fact very low. Using 20 litres of culture merely ca 600 mgr of product could be isolated. No attention is paid to this aspect in the descriptions however. In fact the information derivable from these descriptions would dissuade the skilled person from using this system for producing collagen. As addressed already above this low degree of expression could be due to the repetitivity of the sequence to be transcribed/translated and/or the presence of protease. In particular due to the open structure of non helical collagen any non helical collagen expression product is likely to be extremely susceptible to protease attack.